How do we define the Affinity audiences individual group, eg, who qualify as Social Media Enthusiast

This is a link to the Google Help Doc that explains it really, very well.

"Adding audiences allows you to reach people based on their specific interests as they browse pages, apps, channels, videos, and content across YouTube and the Google Display Network as well as on YouTube search results. You can select from a wide range of categories -- from autos and sports to travel and fashion -- and we'll show ads to people who are likely to be interested. Depending on your advertising goals and the stage of the purchase process your customers are in, you can choose to add different audiences to your ad groups."